Lithium formate nonlinear devices

ABSTRACT

The hydrated and deuterated forms of lithium formate are found to be useful nonlinear materials for use over a wavelength range including the visible spectrum. While conversion efficiency is inferior to that of lithium niobate or barium sodium niobate, resistance to radiation damage is significantly greater. Operation at power levels in excess of 106 watts/cm2 has resulted in no perceptible damage. These materials are suitably incorporated in second harmonic generators as well as in other parametric devices.

United States Patent Bonner et al.

July 25, 1972 LITHIUM FORMATE NONLINEAR DEVICES lnventors: William AdamBonner, Scotch Plains; Shobha Singh, Summit; Le Grand Gerard Van Uitert,Morris Township, Morris County, all of NJ.

Assignees Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

Filed: Aug. 3, 1970 Appl. No.: 60,697

U.S. Cl. ..307/88.3, 321/69 R, 330/45 Int. Cl. ..H03l' 7/00 Field ofSearch ..307/88.3; 321/69;

Primary Examiner-Roy Lake Assistant Examiner-Darwin R. HostetterAttorney-R. .l. Guenther and Edwin B. Cave 57] ABSTRACT 5 Claims, 1Drawing Figure 0mm OF LiCHO H2O PATENTEflJuLzs me 3679 8 CRYSTAL OFLiCHO H2O OR L'LCHOZ-DZO W. A. BONNER INVENTORS 5, S/NGH L. G. VANU/TERT LITHIUM FORMATE NONLINEAR DEVICES BACKGROUND OF THE INVENTION 1.Field of the Invention The invention is concerned with frequencyconversion of electromagnetic radiation over a bandwidth including thevisible spectrum through the use of nonlinear materials. Such conversionmay take the form of second harmonic generation and of other parametricinteractions as, for example, frequency downshifting, frequency mixing,etc.

2. Description of the Prior Art The past decade has seen a significantbody of art develop as a consequence of the invention of the laseroscillator. This source of coherent electromagnetic radiation atwavelengths in and near the visible spectrum has had significant impactin many fields. There are now a variety of operative laser mechanismswhich utilize, inter alia, solid, liquid, and gaseous media. Relateddevelopments have resulted in a variety of devices for controlling andfor modifying the laser radiation.

Many contemplated uses of coherent radiation require a variety ofradiation wavelengths, sometimes a continuum over at least a significantportion of the spectrum. There have been studies and resultantdevelopments designed to meet this requirement in the oscillator itself.A second approach, probably of more immediate practical interest, hasbeen directed toward the development of ancillary elements for changingthe frequency ofa fixed frequency oscillator.

The most promising class of such ancillary elements de pends onnonlinearity, i.e., the nonlinear response of the polarization of themedium, to the amplitude of the wave. Devices based on this phenomenonmay take various form. One form in prevalent use at this time is the SHG(or second harmonic generator). A second form of considerable interestis the parametric downshifter which may be used in combination with anSHG or may be used independently to produce a range of wavelengthslonger than that introduced into this element.

An early nonlinear material and one which is still in use is KDP(potassium dihydrogen phosphate). This material is easily grown in largecrystalline sections of requisite perfection and it has become thestandard by which later developed materials have been judged. One suchlater material is lithium niobate (nominally LiNbO Based on its use asan SHG with a fundamental wavelength of 1.06;; (microns), its conversionefficiency is about two orders of magnitude greater than that of KDP.Incorporation in nonlinear devices has, however, been limited by itssusceptibility to radiation damage (the development of local refractiveindex inhomogeneities which produce light scattering). This damage,which takes place at power levels ofthe order of IO watts per squarecentimeter in a reasonably short period of time, is of consequence evenin CW (continuous wave) use. Since the damage may be annealed out, therehas been some operation at elevated temperatures (above 170 C.) whichavoids the formation of light scattering centers. See Vol. 12, AppliedPhysics Letters, l86 l 968).

Probably the most promising material in CW nonlinear use at this time isa mixed crystal of barium sodium niobate. Its conversion efficiencyissomewhat improved relative to lithium niobate and, more important, it isfree of radiation damage under many operational conditions. Even thismaterial, however. shows damage in pulsed operation at power levels inexcess of watts per square centimeter. The damage in barium sodiumniobate appears to be largely a surface phenomenon, and the possibilityexists that it may yet be avoided even in such high power use byappropriate encapsulation and/or environmental control.

SUMMARY OF THE INVENTION Lithium formate in either its hydrated ordeuterated form has useful nonlinear properties over its transparencyrange which includes the visible spectrum. Crystalline material ofsuitable optical properties is readily grown from aqueous solution.Frequency conversion efficiency is intermediate that of KDP and lithiumniobate (an order of magnitude greater than the former and an order ofmagnitude less than the latter). Most significantly, long-term exposureto pulsed laser radia tion at power levels in excess of i0 watts/cmreveals no radiation damage.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic view of anonlinear device using a material of the invention.

DETAILED DESCRIPTION Composition While modifications are possible, theessential properties of the inventive materials depend upon either LiCHO'H O or LiCHO- D 0. Lithium formate monohydrate or monodeuteratecrystalizes in the orthorhombic system (Class mm 2. space group Pbn)with the lattice constants, c 4.85 A., a 6.49 A. and b l0.0l A. Thereare 4 molecules in the unit cell and the density is L46 g/cm. See I.Nitta, Scientific Papers, Institute of Physical and Chemical Research,Tokyo, Vol. 9, p.l5l (1928).

Possible modifications include substitution of deuterium for thehydrogen bonded to the carbon in the CHO ion.

GROWTH Lithium formate may be grown from aqueous solution, for example,by slow cooling. Resulting crystals may be oriented by X-ray or by othertechniques and may be simply polished with diamond paste. The polishedcrystal may be protected from dehydration either by maintenance in asuitably moist atmosphere or by contacting encapsulation. Illustrativeexamples of growth runs are set forth. Since growth is carried out fromsimple water solution (or D 0 solution) with no additional ingredients,details of growth may vary over a broad range of conditions, forexample, considerable variation is permissible in concentration,temperature, rate of dropping temperature (where such technique isused), use of seeds where desired, etc.

GROWTH EXAMPLEI A glass container of 2 V2 liter capacity was partiallyfilled with water and LiCHO- H O powder was added with stirring at 35 C.until the solution was at or near saturation (the solubility is about 28gms/lOOcc at 20 C. and 40 gms/ l 00cc at 35 C.). A rotating seed holderwith seed crystals attached was introduced and the whole was cooled at0. 1 C. per day resulting in growth of the order of 20 mils per day onthe seed.

GROWTH EXAMPLE II Starting with the same apparatus and startingingredients, a similar growth rate resulted by use of constanttemperature (about 35 C.) while allowing the solvent to evaporate. Forexample, this may be controlled by passing dry N, over the surface in anotherwise sealed container. A typical flow rate for the growth indicatedat 35 C. would be lOcc/hour.

GROWTH EXAMPLE III The deuterated modification was produced by growing acrystal in accordance with Growth Example I except that the startingpowder was first dehydrated by heating to l0O C. for a week in air orinert gas and then dissolving the material in D 0 and growing from D 0rather than H O.

OPTICAL PROPERTIES Lithium formate is essentially transparent from about0.25;; to about l.2u. The principal indices of refraction of lithiumfon'nate as a function of wavelength were obtained by the method ofminimum deviation. A least squares fit of a single term Sellmeierformula to the data yielded the following values for the Sellmeierconstants.

where the subscripts in parenthesis represent the principal axes (seeFundamentals of Optics, F. A. Jenkins and G. E. White, 2nd.Ed.,McGraw-Hill (1950) p.469). The principal axes x, y, and 2 were found tobe parallel to the crystallographic axes a, b, and e, respectively.Using the measured values of the refractive indices at 0.5321 the opticangle 2V was calculated to be l23.72 indicating, that the crystals werenegative biaxial. The measured value of 2V for 0.532lp. light was123.8".

The nonlinear optical properties of lithium formate were investigated bythe method of observing the second harmonic intensities generated usingthe l.064p.m fundamental from a repetitively Q-switched Nd ":YAlG laser.The second order polarizability components for the point group mm2 aregiven y P12: ia r z are the components of the optical electric field ofthe fundamental. From Kelinmans symmetry condition, d d and d d (Vol.l26 Physical Review, p. l,977 (1962)). By observing the second harmonicfringes, the useful nonlinear optical coefficients of lithium formatewere estimated to be several times that ofd of a-quartz.

An examination of index of refraction data and Eq. (I) reveals thatphase-matchable second harmonic generation is possible in lithiumformate for a large number of fundamental wavelengths in the visible andnear infrared.

EXAMPLE LiCHO 'H O was operated as a second harmonic generator in thetype of apparatus schematically depicted in the FIGURE utilizing an NdzYAlG laser operating at 1.06pm. The laser was Q-switched and the pulsedoutput was of a power level of approximately 5 X watts/cm with a pulselength of the order of 0.3 sec. Output was at 0.5321 and operation wascontinued for a period of about 10 hours (approximately lO" pulses).Detailed examination of the crystal after the operation with a view todetecting refractive index inhomogeneities revealed no light scatteringmechanism.

THE FIGURE in the figure, there is depicted a single crystal body ll ofa material of the invention. A coherent electromagnetic beam 12 producedby source 13 is introduced into body 11, as shown. The resultantemerging beam 14 is then caused to pass through filter l5, and, upondeparting, is detected by apparatus 16. For the SHG case, beam 12 is ofa fundamental frequency while departing beam 14 additionally contains awave of a frequency corresponding with the first harmonic of beam 12.Filter 15 is of such nature as to pass only the wave of concern, in theSHG instance that of the harmonic. Apparatus l6 senses only that portionof the beam leaving filter 15.

The device of the figure may similarly be regarded as a three-frequencydevice, with beam 12 containing frequencies to be mixed or consisting ofa pump frequency. Under these conditions, exiting beam 14 containssignal and idler frequencies as well as pump, representing threedistinct values for nondegenerate operation. For any operation, whethertwo frequencies or three, efficiency is increased by resonance. Such maybe accomplished by coating the surfaces of crystal 1], through which thebeam enters and exits. This coating may be partially reflecting only fora generated frequency as, for example, for the harmonic in SHG. For thethree-frequency case, it is desirable to support both generatedfrequencies. in most instances, this cannot be accomplished by coatingthe face of the crystal, and it is necessary to provide at least onespaced adjustable mirror which may be positioned at such distance fromthe face of the crystal ll as to support the frequencies of concern.Simultaneous support of the pump frequency may similarly beaccomplished. However, the complication so introduced is justified onlywhen the pump level requires it.

OTHER CONSIDERATIONS It has been indicated that the crystallinematerials may dehydrate when the relative humidity is very low; forexample, under desiccator conditions. The deuterated form is nodifferent in this respect and both materials must be protected fromextremely low humidity conditions where long-term storage or use iscontemplated. A simple method for protecting the material isencapsulation within a vessel with a suitable H O or D 0 environment. Analternative approach is to coat the surfaces of concern with a suitablytransparent material.

While the main advantage of the inventive materials derives from theirradiation resistance at high power levels, other properties may suggestuse at lower power levels. For example, as compared with lithium niobateand barium sodium niobate, the shorter wavelength absorption edge ofabout 0.25 micron (the other materials have significant absorption atabout 0.4 micron) may suggest CW or pulsed use for redoubling from, forexample, a l.06p. YAG-Nd laser. The broader transparency bandwidth alsosuggests other parametric applications including those at power levelswithin the capability of that of other materials.

While the invention is directed to device use of the describedmaterials, it has not been considered appropriate to this description todescribe suitable device designs in great detail. For example, referenceto means for introducing and extracting radiation may allude simply tothe optically polished surfaces or may include wavelength selectivemedia as, for example, the multiple layer refraction mirrors prevalentin the laser art. Other ancillary apparatus may include wavelengthsensitive deflectors, for example, fixed gratings or acousto-opticelements, etc. Articles describing suitable devices are given in Vol.12,Applied Physics Letters, p.306

(1968). Vol.12, Applied Physics Letters, p.308 (1968) and I Vol.57Proceedings ofrhe IEEE, p.2,096 1969).

What is claimed is:

1. Device comprising a crystalline body consisting essentially of asingle crystal, said crystal being transparent to electromagnetic waveenergy together with first means for introducing a beam of coherentelectromagnetic radiation containing a first frequency component intosaid body and second means for extracting a beam of coherentelectromagnetic radiation containing a second frequency componentdifferent from the first frequency component from said body,characterized in that said body consists essentially of a compositionwhich may be represented by an approximate formula selected from thegroup consisting of LiCl-lO -H,0, LiC- DO -H,O, LiCHO,-D O orLiCDO,-D,O.

2. Device of claim I in which at least one of said means comprises anoptically polished surface.

3. Device of claim 1 in which the birefringence of the said body is atleast as great as the dispersion in velocity of the two said frequencieswithin the said body and in which the said body is positioned so thatthe direction defined by the said incoming beam and the optic axis issuch that an ordinary wave of one of the said frequencies within thesaid body is phase matched to an extraordinary wave of another of thesaid frequencies within the body.

4. Device of claim 3 together with resonant means for supporting astanding wave of at least one of the said frequencies in the said body.

5. Device of claim 4 in which there are coherent electromagnetic wavesof three frequencies within the said body and the frequency relationshipis such that the greatest is equal to the sum of the other two.

I i i i i OLD l: 'UNETED STATES PATENT OFFICE T r 1 w 7) I, CERMMCAM orcommemow Patent No. 3, 679, 90? Dated July 25, 1972 Irwe fls)W.A.Bonner, S.Singh, L.G.Van Uitert It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

001. 3, line 23, delete "where";

line 24, delete "E' line 25, insert -where E's before "are" at the abeginning of the line;

Signed and sealed this 9th day of January 1973.

(SEAL) Attest Z i i EDWARD M. FLETCHER ,JR. ROBERT GOTTSCHALK AttestingOfficer Commissioner of Patents 1 l i i i i 5 i line 26, deleteKelinman's and insert --K1einman's.

FORM po'mso USCOMM-DC wan-poo i U 5 covuvmmv 'IINIINF. HIIIH luv 0- urn:

UNITED STATES PATENT OFFICE CERTIFICATE 0i CORRECTIQN Patent No. 3, 679,90? Dated July 25, 1972 Inventofls) W.A.Bonner, S.Singh, L.G.Van UitertIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below: i

Col. 3, line 23, delete where";

line 24, delete "E' line 25, insert -where E sbefore "are" at the 1 1beginning of the line;

line 26, delete 'Kelinman' s" and insert -Kleinman' s".

Signed and sealed this 9th day of January 1973.

(SEAL) Attest: f I

EDWARD M FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents F ORM PC4050 (10-69) USCOMM-DC 60376-P69 I! S. GOVEINMENIPumnm'. mild in! 0- run

2. Device of claim 1 in which at least one of said means comprises anoptically polished surface.
 3. Device of claim 1 in which thebirefringence of the said body is at least as great as the dispersion invelocity of the two said frequencies within the said body and in whichthe said body is positioned so that the direction defined by the saidincoming beam and the optic axis is such that an ordinary wave of one ofthe said frequencies within the said body is phase matched to anextraordinary wave of another of the said frequencies within the body.4. Device of claim 3 together with resonant means for supporting astanding wave of at least one of the said frequencies in the said body.5. Device of claim 4 in which there are coherent electromagnetic wavesof three frequencies within the said body and the frequency relationshipis such that the greatest is equal to the sum of the other two.